In paper manufacturing processes, it is very important that the mechanical properties of the paper be determined continuously, arid then particularly the strength of the paper in different directions, i.e. the strength anisotropy of the paper. The strength anistropy of paper can be determined when the elastic constants of the paper in different directions are known. It is known that these constants can be determined by subjecting the paper to static forces or to ultrasonic sound.
When the elastic constants are measured with the aid of ultrasonic sound, i.e. mechanical oscillations or vibrations of very high frequencies, e.g. frequencies which exceed 20 kHz, there is utilized the fact that the speed at which the sound propagates in different directions in the material is associated with the elastic constants of the material.
The aforesaid mechanical properties are preferably measured on-line, i.e. directly on the paper web in the paper manufacturing process, while advancing the web continuously, although said properties can also be determined off-line, i.e. on paper samples in a laboratory. Before it is possible to take on-line measurements, it is necessary to solve a number of complicated technical problems which are associated with the specific properties of the material being measured and with prevailing measuring and manufacturing conditions. In this respect, it is necessary to take into account the relatively high speed of the paper web--up to 20 meters per second--and, for instance, the fact that movements are liable to occur in the paper web during its manufacture--web flutter--and that an intensive acoustic noise is generated. Thus, this high noise level coupled with the difficulties associated with exciting the material with ultrasonic waves of sufficiently high energy levels makes on-line measuring of the technical properties of paper very difficult to carry-out with the aid of known technology.
U.S. Pat. No. 4,291,577 describes an arrangement in which a contacting measuring device is used for ultrasound measuring purposes. This measuring device includes a transmitter in the form of a piezoelectric element which generates mechanical oscillations of frequency 20 kHz. Longitudinal high-frequency waves are generated in the excited paper in this way, through the contacting piezoelectric element, these waves propagating in different directions in the material in the plane of the material web. A receiver, which also consists of a contacting piezoelectric element, is located at a predetermined distance from the transmitter, thereby enabling the phase velocity of the ultrasonic sound to be calculated, this velocity being related to the modulus of elasticity of the paper. This arrangement, however, is highly sensitive to external influences, for instance the aforementioned noise, uncontrollable variations in the distance travelled by the web, etc. It will also be evident to all those skilled in this art that an arrangement which utilizes movable parts which are in direct contact, such as the arrangement illustrated in the aforesaid U.S. Pat. No. 4,291,577, will also incur other serious drawbacks. For instance, the known arrangement is relatively complicated, due to its construction, and consequently malfunctions are very likely to occur. Furthermore, it is not certain that the transmitter/receiver will remain in physical contact with the paper web at high web speeds and with paper webs of high surface roughness. The web-contacting parts of the arrangement are also liable to damage the paper.
It is also known to apply the principle of contactless registration of the propagation of ultrasonic sound waves for the purpose of measuring the mechanical strength of a stationary or a moving material. One such method is described, for instance, in Swedish Patent Application No. 8017/70 (Publication No. 359 962). The complicated relationships which prevail between the measured parameters and the elastic paper constants when practicing this method, and also the sensitivity of the method to uncontrollable variations in air flows adjacent the material web, make it difficult to apply this method in practice for on-line measuring processes.
It is also known to utilize bending waves, so-called Lamb's waves to indicate the thickness of and the faults in sheet-like or foil materials. Arrangements of this kind are described in U.S. Pat. Nos. 2,536,128 and 3,210,120, for instance. With these arrangements, energy from a radiation source is fed into sheet-like or foil material at a given angle of incidence, with the aid of a coupling liquid, thereby enabling the phase velocity of the bending wave to be measured. A method which is based on the use of a contact liquid is not suitable for use when measuring or determining the properties of, for instance, paper, for obvious reasons.
U.S. Pat. No. 4,180,324 teaches another method of measuring material properties by studying wave movements originating from an excitation location in the form of ultrasonic sound. Although this method can be used, in principle, for measuring the strength of foil material in a punctiform fashion, it can only be used to determine the strength of the material in its z-direction, i.e. a direction transversely through the material at right angles to the surface thereof. Another distinguishing feature when using the novel inventive arrangement, in addition to not studying the propagation of ultrasonic waves, is that the novel construction affords the possibility of measuring material strength locally in different directions in the plane of the material. This is achieved by utilizing the discovered changeable relationship between the generated macroscopic elastic extension or stretch of the material and the strength of the material in different directions, through geometric configuration and orientation of the excitation zones used.
Another known method described in U.S. Pat. No. 4,674,332 can, in principle, be configured for non-contacting on-line measuring processes. In this case, the ultrasonic sound waves are generated thermally with the aid of laser light. This measuring technique, however, similar to the method according to U.S. Pat. No. 4,291,577, requires accurate determination of running times or phase changes over well-defined travel distances, in order to establish the phase-velocity of the ultrasonic waves generated and propagating in the material. Such measurements are difficult to make accurately, particularly on-line, and the result obtained is also difficult to relate directly to strength properties which are significant to the paper manufacturer, particularly when measuring the properties of paper. One reason for this is that the measurement values obtained constitute the mean values measured over relatively long travel distances, normally tens of centimeters when measuring in the plane of the paper. The occurrence of local minimum values can have a decisive significance on the relationship between the paper strength measured with ultrasonic sound and the paper testing result obtained in the laboratory when using conventional paper testing procedures.